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Cellular Transport Mechanisms

Cellular Transport Mechanisms

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Cellular Transport Mechanisms...
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Cellular Transport Mechanisms:
Living cells constantly interact with the blood to tissue fluid around them, taking in some substances and secreting or excreting others. There are several mechanisms of transport that enable cells to move materials into or out of the cell: diffusion, osmosis, facilitated diffusion, active transport, filtration, phagocytosis, and pinocytosis. Some of these take place without the expenditure of energy by the cells. But others do require energy, often in the form of ATP. Each of these mechanisms is described in the following sections and an example is included to show how each is important to the body.

Example in the body

Movement of molecules from an area of greater concentration to an area of lesser concentration.

Exchange of gases in the lungs or body tissues.
A special form of diffusion involving water moving across a selectively permeable membrane! The diffusion of water.

Absorption of water by the small intestine and kidneys.
Active Transport
Movement of molecules from and area of lesser concentration to an area of greater concentration (requires ATP).

Absorption of amino acids and glucose from food by the cells of the small intestine.
Movement of water and dissolved substance from an area of higher pressure to an area of lower pressure (blood pressure).

Formation of tissue fluid; the first step in the formation of urine.
A moving cell engulfs something.

White blood cells engulf bacteria.
A stationary cell engulfs something.

Cells of the kidney tubules reabsorb small proteins.

Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration (that is, with or along a concentration gradient). Diffusion occurs because molecules have free energy, that is, they are always in motion, The molecules in a solid move very slowly; those in a liquid move faster, and those in a gas move faster still, as when ice absorbs heat energy, melts, and then evaporates. In fig. 3-3, a sugar cube in a glass of water is shown. As the sugar dissolves, the sugar molecules collide with one another. These collisions spread out the sugar molecules until they are evenly dispersed among the water molecules. The molecules are still moving, but as some go to the top others go to the bottom, and so on. Thus, an equilibrium (or steady-state balance) is reached.

Diffusion is a very slow process, but may be an effective transport mechanism across microscopic distances. Within the body, the gases oxygen and carbon dioxide move by diffusion. In the lungs, for example, there is a high concentration of oxygen in the alveoli (air sacs) and a low concentration of oxygen in the blood in the surrounding pulmonary capillaries. The opposite is true for carbon dioxide: A low concentration in the air in the alveoli and a high concentration in the blood in the pulmonary capillaries. These gases diffuse in opposite directions, each moving from where there is more to where there is less. Oxygen diffuses from the air to the blood to be circulated throughout the body. Carbon dioxide diffuses from the blood to the air to be exhaled.

Osmosis may be simply defined as the diffusion of water through a selectively permeable membrane or barrier. That is, water will move from an area with more water present to an area with less water. Another way to say this is that water will naturally tend to move to an area where there is more dissolved material, such as salt or sugar. If a 2% salt solution and a 6% salt solution are separated by a membrane allowing water but not salt to pass through it, water will diffuse from the 2% salt solution to the 6% salt solution. The result is that the 2% solution will become more concentrated, and the 6% solution will become more dilute.

In the body, the cells lining the small intestine absorb water from digested food by osmosis. These cells have first absorbed salts, have become more “salty,” and water follows salt into the cells. The process of osmosis also takes place in the kidneys, which reabsorb large amounts of water (many gallons each day) to prevent its loss in urine.

Active transport requires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of molecules causes them to move to where there are fewer of them. Active transport is therefore said to be movement against a concentration gradient.

In the body, nerve cells and muscle cells have “sodium pumps” to move sodium ions (Na+) out of the cells. Sodium ions are more abundant outside the cells and they constantly diffuse into the cell, their area of lesser concentration. Without the sodium pumps to return them outside, the incoming sodium ions would bring about and unwanted nerve impulse or muscle contraction. Nerve and muscle cells constantly produce ATP to keep their sodium pumps working and prevent spontaneous impulses.

Another example of active transport is the absorption of glucose and amino acids by the cells lining the small intestine. The cells use ATP to absorb these nutrients from digested food, even when their intracellular concentration becomes greater than their extra-cellular concentration.

The process of filtration also requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure. Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure.

In the body, blood pressure is created by the pumping of the heart. Filtration occurs when blood flows through capillaries, whose walls are only one cell thick and very permeable. The blood pressure in capillaries is higher than the pressure of the surrounding tissue fluid. In capillaries throughout the body, blood pressure forced plasma and dissolved materials through the capillary membranes into the surrounding tissue spaces. This creates more tissue fluid and is how cells receive glucose, amino acids and other nutrients. Blood pressure in the capillaries of the kidneys also brings about filtration, which is the first step in the formation of urine.

These two processes are similar in that both involve a cell engulfing something. An example of phagocytosis is a white blood cell engulfing bacteria. The white blood cell flows around the bacterium, taking it in and eventually digesting it.

Other cells that are stationary may take in small molecules that become absorbed or attached to their membranes. The cells of the kidney tubules reabsorb small proteins by pinocytosis, so that the protein is not lost in urine.


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